ELSEVIER Journal of Fluorine Chemistry 80 ( 1996) 95-99

Access to 2-fluoroacrylates and their 3-chloro derivatives by chemical hydrogenolysis of 3,3-dichloro-2-fluoroacrylates

Thoai Nguyen *, Claude Wakselman SIRCOB, Universith de Versailles. Saint Quentin en Yvelines. Equip CNRS Fluor, 45 Avenue de.7 Etuts-Unis, 78000 Versailles, France

Received 2 February 1996; accepted 13 April 1996

Abstract

The formation of 3-chloro-2-fluoroacrylates 2 and 2-fluoroacrylates 3 by hydrogenolysis of 3,3-dichloro-2-fluoroacrylates 1 was studied by using Bu,SnH, zinc, the sodium sulphite/sodium formate mixture or iron pentacarbonyl in the presence of a hydrogen donor (Et,SiH or CH,OH) The two last couples can be used to prepare the 3-chloro derivatives 2, whereas for the preparation of the 3,3-dihydro derivatives 3, zinc is the most appropriate reducing agent.

Keywords; 2-Fluoroacrylate; 3-Chloro-2-fluoroacrylate; 3,3-Dichloro-2-fluoroacrylate; Tcibutyltin hydride; Zinc; Sodium sulphite; Sodium formate; Iron pentacarbonyl; NMR spectroscopy; IR spectroscopy

1. Introduction

Polymethylmethacrylates commonly employed in optical communications have the drawback of having a small trans- parency window (560-670 nm) compared with the broad range of wavelength allowed by the glass and silica fibres [ I]. This is ascribed to the absorption of the overtone of the C-H stretching vibration. Replacement of the hydrogen atom by heavier atoms like deuterium, fluorine or chlorine weakens these absorption bands and consequently improves the optical transparency [ 21. These heavy atoms can be introducedeither in the ester groups or in the vinyl group of the acrylate mon- omers. We report here a method of preparing such monomers; 2-fluoroacrylic acid esters with chlorine and eventually deu- terium incorporated at the 3-position (CXY=CF-COOR, X or Y = H, D, Cl). The choice of fluorine in the 2-position is justified by the outstanding performance known for poly( 2- fluoroacrylates) in optical transmission [ 31 and also by the chemical stability of these polymers. When a fluorine atom is present at the 3-position, nucleophilic attack can occur.

2. Results

Selective reduction of one or two chlorine atoms in 3,3- dichloro-2-fluoroacrylate (1) should lead to the monohydro or dihydro derivative 2 or 3 a shown in Scheme 1:

* Corresponding author.

0022-I 139/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PIISOO22-1 139(96)03486-O

CCl,=CF-COOR - CHCl=CF-COOR -

(1) (2)

CH,=CF-COOR - CH,<HF-COOR

(3) (4)

(a: R=C4H9; b: R=C,Hs)

Scheme 1.

Hydrogenolysis of vinylic chlorine atoms is a relatively easy reaction and numerous procedures are known for this purpose [ 41. In spite of this facility, we have found only one success with this reaction on compound 1 in the literature, i.e. electrolytically [ 51. We have directed our study to various chemical procedures because they can be simple and eco- nomic and they can also be used to prepare the 3-deuterated compounds. We have chosen Bu,SnH, activated zinc, the sodium bisulphite/sodium formate couple or iron pentacar- bony1 in the presence of an hydrogen donor (R,SiH, CH,OH) as reducing agents. The results obtained are summarized in Tables 1 and 3 where the proportions have been determined from 19F NMR spectra.

Ester 1 was synthesized by England et al. in 1958 from dichlorodifluoroethylene and sodium cyanide [6], and by Paleta and Posta in 1967 from 1,2-difluorodichloroethylene [ 71. It can also be obtained from chlorotrifluoroethylene according to our procedure [ 81 (Scheme 2). The vinylic chlorine atoms in the 3-position in compound 1 are activated by the ester group.

96 T. Nguyen, Cl. Wakselman / Journal of Fluorine Chemistry 80 (19%) 95-99

Table 1 Reduction of CCl+IJF-COOC4H9 ( la) (R = C,H,)

Reducing Cont. agent (mol equiv. )

Bu,SnH 2 3

Zn 30 30 30

HCOONa/ 2 2

NaSO,H 3 4 4

Fe(CO)s/ 2 HSiEtJ 2 CH,OH

Temp. Time la Z-2a E-2a 3a (“C) (h) (%I (%) (%) (%)

120 8 21 61 0 18 120 8 12 34 0 54

50 8 22 38 23 17 80 8 0 21 9 70 80 2 45 30 15 10

90 2 45 32 13 10 90 3 21 52 18 9 90 6 0 65 22 13 90 2 16 34 10 19 90 6 0 13 2 19

140 16 324 49 17 0 140 16 64 23 13 0

Observations

21% of 4a 65% of 4a

Table 2 Reduction of 2a and 3a (R = C,H,) by 2 mol equiv. of the NaSO,H HCOONa couple

Compound

CHCLCF-COOC4Hg (ta) CH2=CF-COOC4H9 (3a)

Temp. (“C)

90 90

Time (h)

2 3

Z-2a (%)

27 0

E-2a 3a 4a (So) (5) (%I

4 22 47 0 5 95

Observations

Traces of E

Table 3 Reduction of CCl&F-COOC,Hs (lb) (R = C,H,)

Reducing Cont. Temp. agent (mol equiv.) (“0

Bu,SnH 2 110 5 110

zn 30 80

NaSO,H/ 2 90 HCOONa 4 90

Time lb (h) (%)

3 30 18 0

5 8

3 19 3 0

Z-2b E-2b 3b (%) (%) (So)

56 11 0 45 0 55

45 23 24

32 25 13 0 0 0

Observations

llQof4b 100% of 4b

RONa + CF,=CFCI - ROCF=CFCI z

AgOAc - CCl,=CFCOOR

(1)

(R = alkyl, phenyl)

Scheme 2.

One would expect that even a smooth reducing agent such as Bu,SnH would be able to achieve such hydrogenolysis. The reduction appears to be stepwise with the monohydro derivative 2 being produced first. If less than 2 mol equiv. of Bu&H are used, 2 is the main compound obtained. With more than 2 mol equiv. of reducing agent, the dihydro deriv- ative 3 prevails. By the use of BuJnD [ 91, deuterated 2 and 3 are found.

Activated zinc in DMF [lo] achieves the reduction of compound 1 very easily. Simple heating at 80 “C for 2 h gave 45% of the monohydro derivative 2, 10% of the dihydro derivative 3 and 45% of the starting ester 1. Compound 2 [ 111 is a mixture of Z- and E-isomers in a 2: 1 ratio. If heating was continued for 8 h, the proportion of compound 3 increased to 70%. When a deuterated alcohol like CD,OD was incorporated with DMF, the deuterated esters 2 and 3 were found.

The sodium bisulphite/sodium formate mixture [ 121 also appears to be an efficient system for the hydrogenolysis of 1. With less than 2 mol equiv. of reactant it provides mainly the monohydro derivative 2. Increasing the reaction time increases the yield of 2 but not that of 3. If 3 mol equiv. or more of reactant are used, a third compound 4 is formed at the expense of the expected dihydro ester 3. Compound 4 is a 2-fluoropropanoate. The chemical shifts of its ‘H and “F NMR spectra are in perfect agreement with the values quoted

T. Nguyen, Cl. Wakrelman / Journal of Fluorine Chemistry 80 (I 996) 95-99 91

by Thenappan and Burton [ 131 for ethyl 2-fluoropropanoate. It is likely that 4 is formed from a simultaneous reduction of the double bond of ester 3. In order to check this hypothesis we have carried out two experiments, the results of which are shown in Table 2.

From the results obtained, it seems that the reduction of the double bond of 3 is even easier than the hydrogenolysis of the C-Cl bond of compound 2.

We have also tried the couple of iron pentacarbonyl with a hydrogen donor solvent which has been used successfully by Tanabe et al. to reduce vinyl chlorides [ 141. According to Kruglova and Freidlina who made a thorough study of the couple Fe(CO)S/Et,iH, a radical intermediate is generated from the chloro derivatives [ 151. With compound 1, the monohydro derivative 2 is obtained exclusively. Even under strong conditions ( 140 “C for 48 h), compound 2 was recov- ered unreacted when exposed to this couple. CH30H can also be used as a hydrogen donor in this reaction in place of Et,SiH. This opens the way for the introduction of deuterium by the use of a deuterated alcohol.

Table 3 presents some results obtained in experiments car- ried out on the phenyl ester 1. They are as expected and ensure that the processes used are reliable.

3. Experimental details

NMR spectra were recorded on a Bruker AC300 instru- ment. Chemical shifts are given in ppm, positive downfield from TMS for ‘H and negative upfield from CFCl, for ‘9. Coupling constants J are given in Hz with the usual abbre- viations d, t. q, m, for doublet, triplet, quadruplet and multi- plet. Microanalyses were performed by the Service Central d’Analyse du CNRS at Vernaison. Mass spectra were per- formed by the Laboratoire de Chimie Organique Structurale de l’universite Pierre et Marie Curie de Paris on an AEI-MS 902 spectrometer.

Compounds la and lb were prepared according to a reported procedure [ 83. Bu,SnD and CD,OD were purchased from Aldrich.

The products of reduction, after purification by chroma- tography over silica gel, were analysed by “F NMR spec- troscopy. The proportions of the different components were determined by measuring the integral of the corresponding resonance peaks.

3.1. Hydrogenolysis of ester CCl,=CF-COOC,H, (la) with Bu,Sn H

A mixture of ester la (0.5 g, 2.2 mmol) and Bu,SnH ( 1.2 g, 4 mmol) was heated at 120 “C for 8 h and then distilled rapidly under reduced pressure when 0.8 g of a liquid (b.p. 70-80 “C/3 mmHg) was collected. This was chromatogra- phed on three thin layer plates of Kieselgel 60 using a 1:4 mixture of CH,Cl,/pentane as eluent which left 0.24 g of an oil which was analysed and quantified by 19F NMR spectros-

copy as a mixture consisting of 21% of la of Z-2a and 18% of 3a. An increase in the amount of Bu,SnH led to more dihydro derivative 3a as listed in Table 1. Compounds la and 3a were identified by comparison with known samples [ 7,151 while the Z- and E-isomers of 2a have the following characteristics.

Compound 2a: b.p. 103 “C/33 mmHg. Analysis: Found: C, 46.45; H, 5.55; Cl, 19.65%. C,H’&1F02 requires: C, 46.66; H, 5.55; Cl, 17.72%. ‘H NMR 6: E-isomer: 6.67 (d, lH, CH=, 3JHF= 12 Hz); 4.27 (t, 2H, CH1O, 3Jm.r=7 Hz); I.7 (m, 2H, CH,); 1.4 (m, 2H, CH,); 0.94 (t, 3H, CH,, 3JHu = 7 Hz) ppm. 19F NMR s: - 124.3 (d, CF=, 3Jnr= 12 Hz) ppm. ‘H NMR 6: Z-isomer: 6.8 (d, 1 H, CH=, 3JuF = 20 Hz); 4.25 (t, 2H, CH,, 3JHH=6 Hz); 1.7 (m, 2H, CH,); 1.4 (m, 2H, CH,); 0.94 (t, 3H, CH,, 3JnH = 7 Hz) ppm. 19F NMR S: - 124 (d, CF=, 3JuF= 20 Hz) ppm. IR (CC14) (cm-‘): 1720 (GO); 1620 (C&C).

3.2. Hydrogenolysis of ester la by activated zinc

Zinc was activated according to the method described by Denis et al. [ lo]. In a flask equipped with a reflux condenser were introduced ester la (2 g, 9.3 mmol), activated zinc powder (2 g, 30 matom) and DMF (6 g, 82 mmol). The mixture was heated to 80-90 “C for 8 h when the zinc was separated and washed twice with diethyl ether. The resulting solution was acidified with 120 ml of 30% HCl. The aqueous layer was separated and extracted with ether. The combined ether phase was washed with aqueous sodium carbonate, dried and concentrated. The oily residue collected was chro- matographed on silica gel using a 1:4 mixture of CH,Cl,/ pentane as eluent to give 1.2 g of a mixture of esters la, 2a and 3a in the proportions listed in Table. 1.

The same experiment undertaken with a mixture of DMF (6 g, 82 mmol) and CD,OD ( 1 g, 27 mmol) as solvent left, after chromatography over Kieselgel, 0.8 g of an oil which was analysed by 19F NMR spectroscopy as a mixture of mon- ochloromonohydro ester 2a (Z, lo%, &= - 124 ppm, d, 3J ur=20 Hz; E, 5.5%, &= - 124.3 ppm, d, 3JHF= 11 Hz), monochloromonodeuterio ester (Z, 16%, &= - 124.4 ppm, t,3J,,=3.5Hz;E,5.5%,i&= -124.7ppm,t,3J,,=1.5H~), 11% dihydro ester 3a and 25% monohydromonodeuterio ester, with an equal amount of Z,, and EDF (ZDF, S, = - 117.6 ppm, dt, 3JDF = 1.5 Hz, 3J,F=43 Hz, h5.64 ppm;E,,,&= -117.7ppm,dt,3J,,=5.5Hz,3Ju,=10H~, S, = 5.28 ppm) and 27% of the dideuterio ester ( S, = - 118 ppm, tt, 3JDF = 5.6, 1.5 Hz). Both the slight increase in the chemical shifts of the fluorine atom in the deuterated com- pounds and the feeble coupling constants JDF observed are consistent with previously reported values [ 161. The mass spectrum of the mixture showed a mass of 181 for the mon- ochloromonodeuterio ester.

3.3. Hydrogenolysis of ester la by the sodium bisulphite/ sodium fonnate couple

Into a 100 ml round-bottomed flask fitted with a reflux condenser were introduced ester la (4.2 g, 20 mmol), sodium

98 T. Nguyen, Cl. WakFelman /Journal elf Fluorine Chemistry 80 (1996) 95-99

bisulphite (8 g, 42 mmol), sodium formate (2.4 g, 34 mmol) and DMF (30 g, 410 mmol) The mixture was heated at 90 “C for 2 h, cooled, acidified with 100 ml of 50% HCl and extracted three times with diethyl ether. The combined organic phase was washed with aqueous sodium carbonate, then with water and dried. When concentrated on a rotavapor it left 3.6 g of an oily residue from which 2.5 g of a mixture of esters la, 2a and 3a was separated by chromatography through a silica gel column with a 1:4 mixture of CH,Cl,/ pentane as eluent. With 4 mol equiv. of reducing agent and a longer heating time (Table 1) an additional resonance at - 184.6 ppm was observed in the ‘“F NMR spectrum. This was ester 4a. The characteristics of its NMR spectra are as follows. ‘H NMR 6: 5 (dq, 1 H, CHF, ‘Jnr = 48 Hz, JHH = 7 Hz) ; 4.2 (t, 2H, CH,O, JHH = 7Hz); 1.68 (m,2H,CH,);1.6 (dd, 3H, CH,-CHF, 3JrrF = 24 Hz, Jm, = 7 Hz) ; 1.4 (m, 2H, CH,); 0.93 (t, 3H, CH3-CH2, Juu = 7 Hz) ppm. “F NMR 8: - 184.6 (dq, CFH, *Jur =48 Hz, 3JriF = 24 Hz) ppm. These values are consistent with those reported by Thenappan and Burton for CH,CHFCOOC,H, [ 131.

3.4. Hydrogenolysis of ester la with the Fe(CO)JHSi(Et), couple

A solution of ester la (2 g, 9.2 mmol), Fe(CO), (1 g, 5 mmol) and HSi(Et), (3.4 g, 30 mmol) was heated at 140 “C overnight in a closed Pyrex flask. After cooling on an ice bath, the flask was opened. The slurry was treated with FeC13 (0.8 g, 5 mmol), acidified with 4 ml of 30% HCl, extracted three times with diethyl ether, washed with aqueous sodium carbonate, dried and concentrated. An oily residue ( 1.4 g) remained from which a mixture of esters la and 2a ( 1.1 g) was separated by thin layer chromatography on Kieselgel60. Instead of the silane, methanol ( 1 g, 30 mmol) can be used. The proportion of the esters obtained are listed in Table 1.

3.5. Reduction of ester CH,=CF-COOC,H, (3a) by the sodium bisulphitekodium formate couple

Ester 3a (0.15 g, 1 mmol), prepared according to method [ 171, sodium bisulphite (0.4 g, 2 rnmol), sodium formate (0.14 g, 2 mmol) and DMF (3 g, 41 mmol) were heated at 90 “C for 2 h. Then 15 ml of 30% HCl was added. The slurry obtained was extracted three times with diethyl ether. The ether solution was washed with aqueous sodium carbonate, dried and concentrated. The oily residue was chromatogra- phed on two thin layer plates of Kieselgel60 to give ester 4a.

3.6. Hydrogenolysis of ester CCl,=CF-COOC,Hs (lb) with Bu,Sn H

Using the same procedure as described for ester la, a mixture of ester lb (1 g, 3.6 mmol) and Bu,SnH (2 g, 5.1 mmol) heated to 110 “C for 3 h gave, after chromatography over Kieselgel, 0.4 g of an oil which was analysed as amixture of 30% lb, 56% Z-2b, 11% of E-2b and 3% of 3b. With 5

mol equiv. of Bu,SnH and 18 h of heating, 45% of Z-2b and 55% of 3b were obtained. Z-2b and E-2b have the following characteristics.

Compound 2b: ‘H NMR 6: Z-isomer: 7.08 (d, lH, CHCl, 3Ju,=21 Hz); 7.2, 7.4 (m, 5H, C,H,) ppm. ‘9 NMR 6: - 123.85 (d, F, 3J,,=21 Hz) ppm. ‘H NMR 6: E-isomer: 6.85 (d, lH, CHCl, 3JuF= 12 Hz); 7.2, 7.4 (m, 5H, C,H,) ppm. ‘“F NMR 6: - 124.35 (d, F, 3Jrrr = 12 Hz) ppm. IR (Ccl,) (cm-‘): 1740 (C=O); 1620 (GC); 1590, 1490 (Ws).

3.7. Hydrogenolysis of ester CCl,=CF-COO&Hs (lb) with zinc

Using the same procedure as described for la, a mixture of lb (1 g, 3.6 mmol), activated zinc (2 g, 30 matom) and 2.8 g of DMF heated at 80 “C for 5 h gave, after chromatog- raphy over silica gel, 0.4 g of an oil which was analysed as being a mixture of 8% lb, 45% Z-2b, 23% E-2b and 24% 3b.

3.8. Hydrogenolysis of ester lb by the sodium bisulphite/ sodium formate couple

Using the same procedure as described for la, a mixture of lb (0.5 g, 2 mmol), sodium bisulphite (0.4 g, 2 mmol), sodium formate (0.15 g, 2.2 mmol) and 2.5 g of DMF heated at 90 “C for 3 h left, after chromatography over silica gel, 0.3 g of an oil which was analysed as being a mixture of 19% lb, 32% of Z-2b, 25% E-2b and 13% 3b. With 4 mol equiv. of sodium bisulphite and sodium formate, the main product obtained was 4b (0.1 g) . Compound 4b has the following characteristics.

Compound 4b: ‘H NMR 6: 1.6 (dd, 3H, CH3, 3Jm = 24 Hz, 3Juu = 7 Hz) ; 5.17 (dq, lH, CHF, 3Jm, = 7 Hz, 2Jrir = 48 Hz);7.2,7.4 (m, 5H, C,H,) ppm. ‘“FNMR S: - 148.8 (m, CHF, 2JrrF: = 48 Hz, 3Jur = 24 Hz) ppm.

4. Conclusions

From the reported results we conclude that there are simple ways to prepare 2-fluoroacrylates by chemically reducing the chlorine atoms in the 3-position of esters 1. Activated zinc in DMF appears to be the most appropriate of the four reactants tested. Monohydro derivatives 2 and 3-deuterated com- pounds can be produced if desired. It is interesting that the sodium bisulphite/sodium formate mixture which are com- mon chemicals gives clean products, especially compound 2.

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